Implemented an in-memory window store allowing for range queries. A finite retention period defines how long records will be kept, ie the window of time for fetching, and the grace period defines the window within which late-arriving data may still be written to the store.

Unit tests were written to test the functionality of the window store, including its insert/update/delete and fetch operations. Single-record, all records, and range fetch were tested, for both time ranges and key ranges. The logging and metrics for late-arriving (dropped)records were tested as well as the ability to restore from a changelog.

Reviewers: John Roesler <john@confluent.io>, Matthias J. Sax <matthias@confluent.io>, Bill Bejeck <bill@confluent.io>, Guozhang Wang <wangguoz@gmail.com>